Poly(arylene sulfide/sulfone) polymers are thermoplastic polymers of interest for film, fiber, molding, and composite applications because of their high glass transition points and good mechanical and electrical properties and chemical resistance.
Poly(arylene sulfide/sulfone) polymers can generally be prepared by the reaction of a polyhaloaromatic sulfone with a sulfur source and a polar organic compound, as disclosed in U.S. Pat. No. 4,127,713. In preparations of this type, it is acceptable to have present excess amounts of the sulfur source (e.g. an alkali metal sulfide) in relation to the amount of dihaloaromatic sulfone present. For purposes herein, this preparation will be referred to as a "sulfur-rich" reaction mixture. Such sulfur-rich reaction mixtures typically product poly(arylene sulfide/sulfone) polymers with a characteristic yellow color in contrast to the white color of poly(arylene sulfide/sulfone) polymers produced from reaction mixtures in which excess amounts of dihaloaromatic sulfone is present in relation to the amount of sulfur source present.
One problem experienced with poly(arylene sulfide/sulfone) polymers in the past is that they exhibited relatively low melt stability. When a sulfur-rich reaction mixture is employed, the resulting polymer often exhibits an even lower melt stability due to the presence of mercaptans or metal sulfides as end groups on polymer chains.
One problem frequently encountered in semi-commercial and commercial scale production of poly(arylene sulfide/sulfone) as described, occurs when operating closer to balanced stoichiometry (equimolar amounts of sulfur source and dihaloaromatic sulfone) than the experimental error in reactant analyses and/or charge amounts. Specifically, the aim of a preparation may be to produce the white, more melt stable poly(arylene sulfide/sulfone) by charging an excess of the dihaloaromatic sulfone relative to the sulfur source only to find, when the polymer is isolated, that, due to inaccuracies in the analyses of the concentration of the aqueous sodium sulfide used as the sulfur source, for example, or in the actual charged amounts, that what was actually present in the reactor during polymerization was an excess of the sulfur source relative to the dihaloaromatic sulfone, resulting in less melt stable, yellow poly(arylene sulfide/sulfone).
U.S. Pat. No. 4,774,276 discloses a method to produce poly(arylene sulfide/sulfone) polymers having increased melt stability by treating the poly(arylene sulfide/sulfone) polymers with a zinc source, thereby incorporating zinc cations into the polymer. It would still be desirable, however, to have a method for further improving the thermal stability of a poly(arylene sulfide sulfone) polymer, especially those made using a sulfur-rich reaction mixture.
It is therefore an object of this invention to provide a process for preparing a poly(arylene sulfide/sulfone) polymer having increased melt stability. It is further an object of this invention to provide a process for treating a poly(arylene sulfide/sulfone) polymer prepared using a sulfur-rich reaction mixture in order to improve its thermal stability.